RTD vs Thermocouple – Which is Best?

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Both RTDs and thermocouples are sensors used to measure heat in scales such as Fahrenheit or Centigrade. Such devices are used in a broad range of applications and settings, each with its own advantages and disadvantages.

Resistance Temperature Detectors (RTDs)
The electrical resistance of metals rises as the metals become hotter, and falls as heat decreases. RTDs are temperature sensors that use the changes in the electrical resistance of metals to measure the changes in the local temperature. For the readings to be interpretable, the metals used in RTDs must have electrical resistances known to people and recorded for convenient reference. As a result, copper, nickel, and platinum are all popular metals used in the construction of RTDs. The easiest way to identify an RTD is by its wire leads. RTDs most often have three wires coming out of them, two of the same color and one of a different color, usually two white wires and one red wire. They can be of other colors but these are the type we most often encounter on a turbine. RTDs can have two wires, however they are not often used in industry any longer as they are not as accurate as three wire sensors.

Thermocouples
Thermocouples are temperature sensors employing two dissimilar metals to produce a small voltage that can be read to determine the local temperature. Different combinations of metals can be used in building the thermocouples to provide different calibrations with different temperature ranges and sensor characteristics. They are classified as a “type” of thermocouple such as E, J, K, etc. The different types are made from various types of metals and are used for a wide range of temperatures. The type of thermocouple can only be identified by its lead wire color. Thermocouples always have two wires with dissimilar colors.

RTD vs Thermocouple
Because the terms encompass entire ranges of temperature sensors tailored for use under a range of conditions, it is impossible to conclude whether RTDs or thermocouples are the superior option as a whole. Instead, it is more useful to compare the performance of RTDs and thermocouples using specific qualities such as cost and temperature range so that users can choose based on the specific needs of the application.

In general, thermocouples are better than RTDs when it comes to cost, ruggedness, measurement speed, and the range of temperatures that can be measured. Most thermocouples are half the cost of RTDs. Furthermore, thermocouples are designed to be more durable and react faster to changes in temperature. However, the main selling point of thermocouples is their range. Most RTDs are limited to a maximum temperature of 1000 degrees Fahrenheit. In contrast, certain thermocouples can be used to measure up to 2700 degrees Fahrenheit.

RTDs are superior to thermocouples in that their readings are more accurate and more repeatable. Repeatable means that the same temperatures produce the same readings over multiple trials. RTDs produce more repeatable readings which are more stable, while their design ensures that RTDs continue producing stable readings longer than thermocouples. Furthermore, RTDs receive more robust signals and it is easier to calibrate RTD readings due to their design.

Conclusion
In brief, RTDs and thermocouples each have their own advantages and disadvantages. Furthermore, each make of RTDs and thermocouples possesses its own advantages and disadvantages. In general, thermocouples are cheaper, more durable, and can measure a larger range of temperatures, while RTDs produce better and more reliable measurements.

Overhead Loads

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A young construction worker was killed the same day his wife was coming home from the hospital with their first child. How did this occur? A crane was transporting a heavy, bulky section six or seven feet in the air to clear other objects. The load was guided by tag lines which were used by all of the workers except this young man. Although warned by his foreman to use the line, he didn’t. A lifting pad gave way and he was killed instantly.

Let’s face it, our job is dangerous within itself – we don’t need Murphy’s Law in the mix as well. Be aware of the load at all times, no matter how large or small it is. Remind yourself of this slogan the next time a load is lifted – “IF IT’S IN THE AIR, IT’S DANGEROUS”.

Let’s review some of the rules that can help keep us from getting injured by failing loads:

  • A load that can be carried close to the ground can be stabilized by a person at each end. These individuals must stay in the clear at all times, and the ground surface must be unobstructed and reasonably level.
  • Taglines should always be used where needed and definitely where the load is to be carried more than five feet above the ground. In some cases, ten-foot taglines should be used to guide loads being raised and lowered, rather than using extremely long lines that drag around the job and can snag on something.
  • On all jobs, only one person, generally the lead individual, should give signals to the crane operator. If you are assigned the job of directing the crane, follow these basic rules:
    • Always use standard hand signals to direct the crane operator.
    • Stand in the clear and place yourself where the operator can plainly see you and you can see the operator.
    • If you can’t see the load and another person is signaling to you, be sure everyone is in the clear before you give the signal to the operator. Remember, it takes time to relay signals.
    • Never permit a load to be lowered, raised, or swung over a worker’s head or an occupied building. If the operator can see the load, it’s the operator’s responsibility — without exception — to see that this rule is followed.

REMEMBER …..”If it’s in the air, it’s dangerous.”

Correct Gap Critical to Rotation Speed

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Maintaining the correct gap between the sensor and the rotor is critical to correctly measuring turbine rotor speed. Setting the gap can be problematic if the correct gap is unknown or if it cannot be accessed with a feeler gauge.

First a little background on why the sensor gap is critical: The sensor, or “pickup”, is mounted perpendicular to the shaft, facing a toothed gear fixed to the rotor. Pickups can be either “active” or “passive” (see below). In either case, the pickup counts the teeth by sensing the difference in height between the tip of the tooth and the valley between the tooth. If the sensor is too close, it can’t reliably distinguish between the tip and the valley. If it is too far away, it can’t reliably register the tip. The correct gap will register a voltage differential which can be counted. The electronic circuits determine shaft rotation speed by dividing the number of teeth on the gear into how fast the voltage changes over a period of time.

Always use the manufacturer’s specification data to determine the correct gap spacing for the pickup. If the specification is not available, an initial setting of 0.025″ (0.64 mm) will work in most cases. Once the unit is running, the controls engineer can determine if the output voltage or signal level is sufficient for the type of control used.

Sometimes the pickup gap cannot be accessed with a feeler gauge. If so, an accurate setting can be obtained with a little math and a technique called “counting the flats”. The two most often found pickup sizes are the 5/8″ – 18 and the 3/4″ – 20 thread sizes. If you do the math, the 18 threads per inch (TPI) device will move 1 inch in the mounting hole if it is rotated 18 times. Looking at it the other way, it will move in the hole 0.055 inches if it is rotated one time. Breaking it down further, it will move 0.009″ if it is rotated one “flat” of the hexagonal shaped body or hex nut. In the case of the larger 3/4″ inch device, it will move 0.050″ per one rotation and 0.008″ per “flat”.

To “count the flats”, line up the tooth of the gear as close as possible to the center of the mounting hole until it looks like the picture above. Once the gear tooth is aligned with the center of the hole, screw the pickup down BY HAND until the face of the pickup gently contacts the tooth. Set the gap by unscrewing the pickup while counting the flats from a fixed reference point (can even be a line made by a Sharpie pen). For a 0.025″ gap unscrew it by 2 ¾ flats. The math would be 0.025″ (gap) / 0.009″ (movement per flat) = 2.77 flats or approximately 2 ¾ flats. For the larger pickup size that would be 0.025″ / 0.008″ = 3.1 flats or just tad over three full flats. Tighten up the locknut and you’re done!

There are many different types of pickups out in use today but the most common types used on steam and gas turbines are the “passive” type (sometimes called inactive pickups) and the “active” type. They look very similar but operate quite differently. Very simply, the typical magnetic or “passive” pickup is simply a coil of fine wire wrapped around a magnetized iron core that self generates a voltage. When the tooth of a gear passes in front of the iron core, a small voltage is generated and when the valley between the teeth passes the iron core, the voltage falls off. The “active” type pickup receives power from an outside source instead of self generating it. There is a small transmitter and receiver inside of the device that sends out a signal from the end of the pickup. When a gear tooth passes this signal, it changes the characteristic of the signal that is reflected back to the receiver. The internal electronics then interpret this and send out a voltage pulse. Although the passive sensor generates a sine wave and the active sensor generates a square wave, both sensors count cycles over time, which represents teeth rotation speed.

Please contact Mr. Turbine® for answers to any issue with Steam or Combustion Turbine Controls, or Generator and Exciter Controls for any motive power system.

Heat Stress

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 It’s that time of year again – summertime – hot and unbearable weather. This year is starting off in pretty much the same pattern as years gone by. All over the U.S we are seeing extremely high heat temperatures, especially in the western portion where temperatures are reaching an average of 112° to 128° degrees. Many people are exposed to heat on some jobs, outdoors, or in hot indoor environments. Operations involving high air temperatures, radiant heat sources, high humidity, direct physical contact with hot objects, or strenuous physical activities have a high potential for causing heat-related illness.

Why is heat a hazard to workers?

When a person works in a hot environment, the body must get rid of excess heat to maintain a stable internal temperature. It does this mainly through circulating blood to the skin and through sweating.

When the air temperature is close to or warmer than normal body temperature, cooling of the body becomes more difficult. Blood circulated to the skin cannot lose its heat. Sweating then becomes the main way the body cools off. But sweating is effective only if the humidity level is low enough to allow evaporation and if the fluids and salts that are lost are adequately replaced.

If the body cannot get rid of excess heat, it will store it. When this happens, the body’s core temperature rises and the heart rate increases. As the body continues to store heat, the person begins to lose concentration and has difficulty focusing on a task, may become irritable or sick, and often loses the desire to drink. The next stage is most often fainting and even death if the person is not cooled down.

Excessive exposure to heat can cause a range of heat-related illnesses, from heat rash and heat cramps to heat exhaustion and heat stroke. Heat stroke can result in death and requires immediate medical attention.

Exposure to heat can also increase the risk of injuries because of sweaty palms, fogged-up safety glasses, dizziness, and burns from hot surfaces or steam.

How can heat-related illness be prevented?

Heat-related illnesses can be prevented. Important ways to reduce heat exposure and the risk of heat-related illness include engineering controls, such as air conditioning and ventilation, that make the work environment cooler, and work practices such as work/rest cycles, drinking water often, and providing an opportunity for workers to build up a level of tolerance to working in the heat. Employers should include these prevention steps in worksite training and plans. Also, it’s important to know and look out for the symptoms of heat-related illness in yourself and others during hot weather. Plan for an emergency and know what to do – acting quickly can save lives!
Remember, refrain from alcohol intake the night prior and drink plenty of fluids during the shift.

Common Sense Safety

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Contrary to popular opinion, all workers can prevent themselves from getting hurt. The easy way to avoid pain is to observe how others have taken risks and been injured, rather than learning the hard way–from your own injury. That’s common sense! By avoiding unsafe acts and practicing common sense, your work will go smoother, with less chance for accidents.

The experts say at least 80% of industrial accidents are caused by unsafe acts on the part of employees–and not by unsafe conditions. Although employers are required by law to provide a safe and healthful workplace, it is up to you to be aware of your work environment and follow safe work practices. Statistically, most accidents are caused by unsafe acts, including:

Being In A Hurry – Sometimes there is more concern for completing a job quickly instead of safely. Take time to do a good job and a safe job.

Taking Chances – Daring behavior or blatant disregard for safe work practices can put the whole work team at risk. Follow all company safety rules and watch out for your fellow employees. Horseplay is never appropriate on the job and can lead to disciplinary action.

Being Preoccupied – Daydreaming, drifting off at work, thinking about the weekend, and not paying attention to your work can get you seriously hurt or even killed. Focus on the work you are paid to do. If your mind is troubled or distracted, you’re at risk for an accident.

Having A Negative Attitude – Being angry or in a bad mood can lead to severe accidents because anger nearly always rules over caution. Flying off the handle on an outage is potentially dangerous. Keep your bad moods in check, or more than one person may be hurt. Remember to stay cool and in charge of your emotions.

Failing To Look For Hidden Hazards – At many jobsites, work conditions are constantly changing. Sometimes new, unexpected hazards develop. Always be alert for changes in the environment. Hidden hazards include spilled liquids that could cause slips and falls; out-of-place objects that can be tripped over; unmarked floor openings one could step into; low overhead pipes that could mean a head injury; and other workers who don’t see you enter their hazardous work area.

Remember to stay alert for hazards, so you won’t become one more accident statistic: You can do a quality job without rushing. Maintain a positive attitude and keep your mind on your work. This is just common sense–something smart workers use!

Copper Resistance Testing

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General. The typical generator stator winding consists of three independent circuits or phases. These circuits must be as symmetrical as possible with one another to achieve maximum generator efficiency. This symmetry includes the cross section of each conductor as well as the length or distance from main lead to neutral lead. The Copper Resistance (or Winding Resistance) test measures this “distance” for proper length and high resistant anomalies (i.e. cracks, shorted turns, open circuits, etc.). An increase in resistance from one test to another could indicate the potential for a future winding failure.

Test Setup & Execution. The main and neutral lead connections should be broken and open. The lead ends should be free and clean of surface contamination so that the test probes make good contact. Copper resistance testing is performed with a Digital Low Resistance Ohm Meter (DLRO) test set. The DLRO instrument is extremely sensitive. Poor contact and circuit set-up can either produce erroneous readings or no readings at all.
One probe of the DLRO is connected to one lead of an individual phase, and the other probe connected to the other lead of the same phase. A reading (generally to the fourth decimal place) in ohms resistance is measured and recorded. This same process is repeated on the second and third phase. The ambient air temperature and humidity should be recorded as well.

Interpretation of Results. Temperature significantly influences the resistance of a dielectric as well as a conductor. For this reason, the copper resistance measurement should be corrected to standard (typically 40°C).
The original equipment manufacturer normally records and documents the as-built phase-by-phase copper resistance measurements. These are used as the baseline by which all future readings can be compared.
An increase in in copper resistance indicates the presence of some form of high resistance issue (i.e. broken conductors, cold braze joints, turn-to-turn shorting, incorrect connection, incorrect number of turns or stranding, open circuit). Additional testing will be required to determine the specific cause of the variant reading.

Standard. IEEE Standard 11 8TM-1 978, IEEE Standard Test Code for Resistance Measurement.

Test Equipment. A Megger, Model DLRO-10 or comparable is recommended. Kelvin and Wheatstone bridges are also used to measure resistance.

Slips, Trips, and Falls

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Falls are one of the leading causes of unintentional injuries in the United States, accounting for nearly 8.6 million visits to the emergency room. Adults 55 and older are more prone to being victims of falls, and the resulting injuries can diminish the ability to lead active, independent lives. The number of fall deaths among adults 65 or older is four times the number of fall deaths among all other age groups.

Most common locations for falls:

  • Doorways
  • Ramps
  • Cluttered hallways
  • Areas with heavy traffic
  • Uneven surfaces
  • Areas prone to wetness or spills
  • Most common locations for falls to another level:
  • Unguarded heights
  • Unstable work surfaces
  • Ladders
  • Stairs

Falls can be prevented – Ladder Safety

  • Always keep at least three points of contact with the ladder (i.e., two hands and one foot or two feet and one hand)
  • Place the base on a firm, solid surface
  • A straight or extension ladder should be placed 1 foot away from the surface it rests against for every 4 feet of ladder height. When you climb, always face the ladder and grip the rungs, not the side rails
  • Climb down a ladder one rung at a time
  • Do not climb with tools in hand – use a tool belt
  • Keep your body between the ladder side rails when climbing. Do not lean or overreach – reposition the ladder closer to the work instead
  • Do not use ladders outdoors in windy or inclement weather

Casing Repair – Part 3: Distortion & Erosion

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The final Turbine Generator Tip in this series discusses two common steam turbine casing problems – Distortion and Erosion. The repair methods employed – grinding, mechanical repair, welding and stress relief – have their own set of considerations which were covered in previous portions of the series.

Casing Distortion
Casing Distortion becomes a strong likelihood when the units accumulate operating cycles. The most common causes of distortion are steady state and transient thermal stresses which can occur within all turbine sections (HP, IP, LP). Inner casings distort more easily than outer casings due to their thinner cross-section and higher temperature differentials across the casing walls. Distortion typically causes problems during disassembly and reassembly. Some examples of this are bolting interferences, gaps at the horizontal joint, galling of the fits and misalignment of the steam path seals. These problems can lead to steam leakage and rubbing. Internal leakage due to distortion reduces efficiency and power output, while leakage to atmosphere and internal rubbing can both cause a forced outage.
Water induction can cause extreme distortion of the inner cylinders. This can damage internal steam path components and lead to forced outages. Inner casings as well as valve bonnet covers can become severely warped and may require extreme measures to remove and replace.
Casing distortion can be corrected by welding, machining, localized heating and rounding discs inserted during stress relief. See previous Tips in the series for considerations in employing these methods.

Erosion
Damage from erosion affects different designs at different locations, but both rotating and stationary components are vulnerable. Erosion typically takes place in the LP section where steam enthalpy drops below the saturation point. Crossover pipes and inlet areas to the LP section could increase in roughness as the surfaces wear unevenly. Support struts may thin or be cut through. Moisture erosion can also take place in the exhaust ends of HP and IP sections if the turbine operates for long periods at low load or goes through frequent start-ups. Horizontal joints may erode and leak between stages and stationary blade support rings may erode as well as crack.
Casings, diagrams, hoods and crossovers are usually made of carbon steel or cast iron. These materials erode approximately 20 times faster than blading material made out of 400 stainless steel.
Erosion can contributes to major damage. Repairs must be aimed at improving the erosion resistance of the steam path and support surfaces. Methods also must be examined for reducing steam moisture content and the size of droplets.
Eroded areas can be rebuilt. Stainless steel or other erosion resistant weld metal can be applied to eroded seal surfaces such as horizontal joints, flow guides and diaphragm inner and outer rings and joints. Fabricated stainless steel liners can be welded inside of crossovers, seal areas and inlet flow areas of casings. They may also be applied over support struts to protect the existing cast iron, steel or low alloy castings. No stress relief is required in most welding applications. Epoxy or ceramic coatings may be suitable for surfaces that are not suitable for weld overlay.
For more information on your particular application, please contact Mr. Turbine®.

Casing Repair – Part 2: Welding Considerations

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Welding is a common method to repair turbine casing cracks, but it must be applied with consideration. Most turbine casing alloys can be welded using either of two distinct procedures: stress relieved and non-stress relieved. The procedure selected is often dictated by time and cost restraints.

Non-stress relieved welds have the advantage of lower cost and shorter outage time. The disadvantage is that the weld can be short lived. The procedure follows this outline: A preheat of about 500 degree F or greater is used. A shielded metal arc weld is performed with a non-matching high nickel content filler. This use of dissimilar metals as filler can lead to low cycle metal fatigue. No post-weld stress relief is performed but the preheat conditions are maintained throughout the process.

Stress relieved welding offers the best potential for a long repair life, but is complicated and time consuming. The procedure follows this outline: A lower preheat of about 300 degree F is used. A shielded metal arc or metal inert gas weld is performed with a matching metal content filler. The casing is then placed in a furnace and raised to a temperature of over 1,000 degrees F. The exact temperature depends on the alloy, the procedure and the application. Much higher temperatures may be required. There are no problems with differential expansion during turbine operation since the weld uses matching filler metal.

The pre-weld residual stress levels in the casing must be carefully assessed to increase the probability of a successful weld. The high levels of residual stresses in the casing can combine with the added stresses of welding to cause uncontrolled distortion and hot cracking during the stress relief phase. Residual stresses generated by the weld passes can be reduced through techniques such as grinding, peening between passes, and peening and grinding. Therefore, the welding procedure must be performed by a skilled contractor.

The best way to control distortion during stress relief is to bolt the casing halves together and place the assembly in the furnace. This would be most applicable to an inner casing that can be easily removed from its outer casing. If only the upper half of the casing is going to be repaired, a thick plate can be bolted onto the horizontal joint as a substitute for the lower case. Distortion can be further controlled by inserting custom fabricated rounding rings or disks into the assembly before thoroughly bolting it together.

If the facility has ample room, a portable furnace can be built on-site. Otherwise, the assembly must be sent out for this process. If the assembly is too large for the furnace, stress relief can be done on a local area of the case, allowing suitable temperature gradients away from the weld areas. Whatever the location, the temperature of the furnace and the assembly must be stringently monitored during the entire stress relief process. Multiple heat cycles and possible re tightening of the joint bolting between cycles may be necessary. This is a process which has been refined over the years and continues to get better. Again, it is always a good practice to perform an assessment prior to performing any of the above procedures.

The next Turbine Generator Tip in the series discusses casing distortion and erosion problems. For more information on your particular application, please contact Mr. Turbine®.

Knife Safety

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Statistics indicate that knives cause more disabling injuries than any other hand tool. People in all occupations are injured by knives – a stock boy in a supermarket produce department or a mechanic opening a box or cutting rubber. We are all exposed to knife injuries only because a knife is a very handy and commonly used tool.

All cuts should receive first aid. Even the smallest cut can become infected, so treat all cuts properly. Always use a knife only for what it is intended. Never use it as a screwdriver or pry bar. Never use a knife that is defective. Keep knives sharp and in good condition. A dull knife can cause you to put too much pressure on the object you are trying to cut. The blade could slip and slice you or someone nearby.

The principal hazard when using a knife, whether on or off the job, is that the user’s hand may slip from the handle onto the blade, causing a painful and serious injury. A handle guard will reduce this hazard. Another cause of injury is the knife striking the free hand or the user’s body.

Industrial knife safety principles remind us to always make a cutting stroke away from the body when possible. Adequate protection should be worn to protect the body and provisions made to hold the material steady. Steel-mesh gloves are available in select industries, such as meatpacking, where materials must be held in close proximity to the knife. TGM carries these steel mesh gloves in every tool set we own. We are in the process of getting Kevlar gloves as well.

When on the job, carry a knife in a sheath or holder over the right or left hip, pointing backwards. Otherwise, a fall could cause a serious leg injury. Storage of knives is also an important safety factor. Cutting edges should be covered and not exposed. Knives should be kept in their proper place and not left on benches or on the floor.

If you are using the right knife for the job, it should cut without great difficulty. When you have to resort to force to make a knife cut, then you are headed for trouble–it could result in an injury to you, damage to the knife, or damage to the material that you are attempting to cut. Remember this, “our patience will achieve more than our force.” That is a good point to remember when using a knife.